ZFS and Thin Provisioning

12 Aug '06 - 04:39 by benr

ZFS is an amazing and wonderful technology. I say technology because its more than just any one of its capabilities. Being able to dish out, from a single pool, both filesystems and traditional volumes (which I'll call zvol's) makes for an extremely power storage foundation on which to build monumental structures without the traditional complexity that comes from such beloved products as my old friend Veritas Volume Manager (VxVM). In a world in which storage design and management only seemed to get more and more complex, a calm and peaceful breeze has come over the landscape and refreshed all of us baking under the heatlamp of rusty and incapable software. Yes, ZFS makes me happy, very very happy indeed.

When ZFS appeared on the scene a while back there was such an outpouring of blogging about it, particularly from its creators themselves, that I felt unable to really add anything to the conversation and instead picked some small topics to help fill in the discussion, such a Look ma' no disks! ZFS Testing On A Budget entry in which I highlight a storage-admins dream feature, ZFS architecting without having to own a Thumper or large disk sub-system when you don't need capacity, you just need a big disk count. If you feel like your little single-disk workstation isn't sufficient to really enjoy and play with ZFS's capabilities, please read that entry!!! Don't get left out of the fun.

Okey, intro out of the way, lets get on with it... look at my "little" home dev workstation:

So, then, since I said that this is on a "little" workstation, how did I do it? The topic gives it way, the answer is thin provisioning, or more properly as its called by ZFS, sparse volumes.

If its not yet obvious, what is happening here is that I'm allowing ZFS to "fake out" the size fo the volume that I created. The real disk available is a rather wimpy 53GB. So whats the point of faking out a volume? Here's the problem with traditional "scalable" storage: you buy 1TB of disk, you create a 1TB filesystem on it, and your happy for a time. One day you hit 90% on the filesystem and like a good storage consumer you buy an additional expansion array, bolt it on and increase the size of the volume up to 2TB. At this point you need to increase the size of your filesystem to match the 2TB available.... growing a filesystem in most cases is a real painful and scary process. At best your going to write-lock the filesystem during the growing operation, at worst you need to move out your data and then restore it back in after a fresh newfs. Almost all modern filesystems allow you to grow them and most allow it online so the backup and restore upgrade isn't often an issue anymore, but its still a sucky process.

Filesystem suckiness aside, how often are you actually using all the blocks you allocate? And how often are you buying more disk than you need just so that you don't have to allocate more in the nearish future? Constraints constraints!

And so for these, and yet even more, reasons, sparse volumes, better known as "thin provisioning", allows us to set advertised volume size to almost anything we want and to use it just as if it actually were that large. That means that in the future when we get close to the capacity of the real disk we can simply add another disk to the pool to meet the demand and, this is the beauty part, your done! Because you've already sized the end filesystem just adding disk to the pool is all thats requried, meaning no down time, no confusion, no fear, and best of all, your users and customers think that you invested in a, say, 10TB storage-subsystem when in fact you've only got a 500GB drive connected via USB 2.0. :) w00t!

Okey, so how do you do it. Simple, you create a zvol with the -s, for "sparse", and set the volume size to whatever you want. Observe:

There are two important values to consider, both of which can be viewed with zfs get var pool/fs: volsize and reservation. Notice the diffrence between zvol1 which is a reserved 10G and thinvol1 which is an un-reserved 100GB, which are, mind you, in the same 88GB pool:

Because the reservation is disabled ("none") we can define volsize as large as we want, whenever we want, in fact, right on up and beyond my not so wimpy 4TB.

While this might seem like a simple feature at first, not worthy of all the reading you've done to arrive at this very point, trust me, when you pair this fine wine with a good cheese, say iSCSI, you can do some very interesting and exciting things and make your life a whole lot less stressful, all without having to shell out $50,000+.

- - C O M M E N T S - -

For all of the reasons you mentioned, that’s simply really exciting.

One of the futures I see for ZFS technology is in consumer PCs.

“WHAT?”

Everyone thinks ZFS is a server technology, and it is, but look at it from a consumer point of view.

Rather than having C:, D: , etc. etc. or /vol1 /vol2, you instead have /disk.

Period.

One seameless, endlessly expandable blob of storage.

Your disk fills up, you need a new drive. Get the new drive, if you want to keep all of your photos in MyPhotos, well, you’re pretty well stuck unless you free stuff up on that volume.

WIth ZFS, add a new drive, and voila! MyPhotos is now 250G bigger, just like everything else. No copying, no soft-link chicanery, no “short cuts”. Nothing.

Combine the simplicity of that kind of management with not having to “resize” your “old” 250G filesystem, and it makes the process of adding storage to systems just that much easier.

That makes the “computer” part of computer just that much easier to use.

Your illustration of Thin provisioning in ZFS is excellent to say. Thin provising at the moment is not catching much attention on any sys admin because many do not really understand it’s benefit when they run out of disk space on the front end and the pain involves in taking care of data that might get interupted in the volume expansion process.

Bernd: As I noted, its most compelling when mixed with another technology… namely iSCSI. Imagine ext3/xfs/jfs on iSCSI… or even NTFS.

If you don’t have an imagination, then clearly you shouldn’t be using sparse volumes, but for storage architects with vision we need all the tools we can get our hands on to build the solutions that are just what we need… thin provisioning is just one of those tools.

benr - 14 August '06 - 03:01

@Ben: yes I see the afvantage of on-demand resizing (well growing) a File system with centralized Storage (whats wrong with FC?), but why do I need to have a sparse Volume for that? I mean I just can grow the Filesystem as easyly, right?

(Maybe I am just unexperienced but I never saw a half empty file system besides the one which needed the spindles)

Question for you Ben. How does ZFS protects against an unexpected flurry of writes against Thin Provisioned storage? How am I protected? Is there any automatic risizing that can be done. Thin provisioning can be risky especially when most storage allocations are based on forecasting which is risky at best. How does ZFS mitigates this risk?

Thanks
Pq65

Pq65 - 19 August '06 - 20:19

Sadly, Solaris iSCSI target is totaly unusable with sparse volumes. First thing it does after creating new target is filling it with zeros, effectively exhausting all backing storage of sparse volume.

Just a question of interest. I have a E450 attached to SE 3510 with Veritas 3.4 volume manager installed on Solaris 8. Now I want to upgrade solaris 10 and then move from veritas volume manger to ZFS. What will be the best way to approach this. Thanks

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I have a E450 attached to SE 3510 with Veritas 3.4 volume manager installed on Solaris 8. Now I want to upgrade solaris 10 and then move from veritas volume manger to ZFS. What will be the best way to approach this.

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on any sys admin because many do not really understand it’s benefit when they run out of disk space on the front end and the pain involves in taking care of data that might get interupted in the volume expansion process.